1. Field of the Invention
The present invention relates to a distortion compensation address generator, a distortion compensating circuit, and a transmitter.
2. Description of the Related Art
In communication apparatuses such as a cellular phone, etc., as shown in FIG. 17(a), a signal is radio transmitted from an antenna 503 through amplification due to an IF amplifier 501 and a transmitting amplifier 502. As an I/O (input/output) characteristic of the transmitting amplifier 502, it is ideal that a good linearity as shown in a dot-dash line of FIG. 17(b) is ensured. However, in actual, as shown in a solid line of FIG. 17(b), an amplification factor is reduced as input signal voltage increases higher because of a characteristic of a semiconductor element forming the transmitting amplifier 502, with the result that a non-linear distortion occurs. This distortion causes deterioration of communication quality.
For this reason, in order to compensate for the non-linear distortion of the transmitting amplifier, there is provided a circuit (distortion compensating circuit) 500 for compensating for the distortion at a digital data stage as shown in FIG. 17(a).
The distortion compensating circuit 500 compensates for transmitting data using compensation data having a characteristic opposite to the actual I/O characteristic of the transmitting amplifier (characteristic shown in the solid line of FIG. 17(b)), so that the linearity of transmitting amplifier 502 is improved.
Since the value of transmitting data varies instantaneously in real time, the compensation of the non-linear distortion of the transmitting amplifier must be performed to follow the variation at high speed.
Also, in mobile communication, transmission power control is carried out for the purpose of reducing co-channel interference and power consumption of the mobile apparatus. If a gain of the transmitting amplifier is adjusted by the transmission power control, the compensation for the non-linear distortion must be carried out to follow the adjustment appropriately.
In FIG. 17(a), the reason why a transmission power control signal is input to not only the transmitting amplifier 502 but also the compensating circuit 500 is that the distortion compensation must be carried out in accordance with transmission power.
As a useful system, which realizes a high speed distortion compensation, there is a system in which distortion compensation data is stored in a memory and an memory address is generated at high speed by address tables and distortion compensation data is output from the memory, that is, a memory access system using the address tables.
However, in the memory access system using the address tables, numerous comparators, which are operated in parallel, are required, and these comparators must be operated at high speed to follow the change in transmitting data, with the result that there is difficulty in downsizing the communication apparatus and obtaining low power consumption.
Also, for compensating for the non-linear distortion of the transmitting amplifier in mobile communication, particularly a cellular phone, transmission power of the transmitting amplifier varies over several tens of gradations (levels) by transmission power control. Therefore, the number of address generation tables corresponding to the number of gradations must be prepared. Moreover, in order to perform high-speed distortion compensation, it is necessary to operate the plurality of address generation tables. Therefore, in view of this point, there is difficulty in downsizing the communication apparatus and obtaining low power consumption.
The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to realize miniaturization of the circuit and low power consumption as ensuring high-speed distortion compensation of the transmitting amplifier.